Banca de QUALIFICAÇÃO: VINICIUS GOMES DE SOUSA DUARTE

Synthesis and Characterization of NiNb2O6 Obtained from the Oxalic Precursor of Niobium for Application in Electrocatalysis

 nickel niobate; niobium oxalate precursor; solid-state reaction; band gap;electrocatalysis

Nickel niobate (NiNb2O6) has attracted increasing interest due to its structural,physicochemical, and electrocatalytic properties, showing great potential for energy conversion and storage applications, particularly in lithium-ion batteries and  electrocatalytic hydrogen production. In this context, the present work investigatedthe synthesis and characterization of NiNb2O6 obtained by the conventional solid-state reaction route using ammonium niobium oxalate [(NH4)3NbO(C2O4)3·nH2O] as the niobium precursor, aiming to reduce the crystalline phase formation temperature and promote the production of nanostructured materials. The oxalate  precursor was initially prepared from commercial Nb2O5 through fusion, washing, and complexation steps, and subsequently mixed with nickel oxide followed by high-energy ball milling, compaction, and calcination at temperatures ranging from 600 °C to 1100 °C. For comparison purposes, an additional sample was synthesized using commercial Nb2O5 as the niobium source. The obtained materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDS), UV–Vis spectroscopy, and thermogravimetric analysis (TGA) of the niobium oxalate precursor. The results demonstrated that the oxalate precursor promoted the formation of single-phase orthorhombic columbite-type NiNb2O6 from 900 °C onward, suppressing undesirable secondary phases and enabling phase formation at lower
temperatures than those required for synthesis routes based on commercial Nb2O5. FESEM micrographs revealed fibrous morphologies with porous agglomerates at lower calcination temperatures (600–700 °C), whereas elongated and irregular nanostructures, typically reported for niobates, predominated between 800 °C and  900 °C. At 1000 °C and 1100 °C, grain growth and increased densification associated with pre-sintering effects were observed. These findings indicate that the use of the oxalate precursor is a promising strategy for synthesizing nickel niobate at reduced temperatures due to its high reactivity, yielding materials with morphological and structural characteristics favorable for catalytic applications. Furthermore, UV–Vis analyses revealed band gap values between 2.0 and 2.5 eV for the samples calcined at 600 °C and 1100 °C, in agreement with the literature for nickel niobate, corresponding to a relatively narrow band gap compatible with light absorption in the visible and ultra violet regions.